This invention relates to thickener/clarifier settling tanks used to separate liquid and solid components of an influent feed slurry and specifically relates to feedwell apparatus employed in such thickener/clarifiers to enhance the clarification process. More specifically, the invention relates to a feed dilution system that delivers a diluted solids slurry to a feedwell of a thickener or clarifier tank.
Thickener/clarifier settling tanks are used in a wide variety of industries to separate influent feed slurry comprising a solids, or particulate material, containing fluid to produce a “clarified” liquid phase having a lower concentration of solids than the influent feed slurry and an underflow stream having a higher concentration of solids than the influent feed slurry. Thickener/clarifier tanks conventionally comprise a tank having a floor and a continuous wall, which define a volume within which the clarification process takes place. Thickener/clarifier tanks also include an influent feed conduit for delivering influent feed to the tank, an underflow outlet for removing settled solids from the tank and a fluid discharge outlet for directing clarified liquid away from the tank. Thickener/clarifier tanks may also include a rake assembly having rake arms for sweeping along the floor of the tank and may further include an overflow launder or bustle pipe for collecting clarified liquid near the top of the tank.
Thickener/clarifier settling tanks of the type described operate by introducing an influent feed stream into the volume of the tank where the influent is retained for a period long enough to permit the solids to settle out by gravity from the fluid. The solids that settle to the bottom of the tank produce a sludge or solids bed near the bottom of the tank, which is removed through the underflow outlet. Clarified liquid is formed at or near the top of the thickener/clarifier tank and is directed away from the tank for further processing or disposal. Settling of solids may be enhanced in some applications by the addition of a flocculant or polymer that forms agglomerates that settle more readily. In many applications, an objective of fluid clarification is to enhance the settling process to achieve a high throughput of solids, and thereby enhance solids recovery.
Many thickener/clarifier settling tanks are constructed with a feedwell, usually centrally located within the tank, into which the influent feed slurry stream is delivered. The feedwell generally serves the purpose of reducing the fluid velocity of the incoming influent feed stream so that the energy in the stream may be dissipated to some degree before entering the tank. Dissipation of energy in the influent feed stream lessens the disruptive effect that the incoming influent feed stream has on the settling rate of the solids in the tank. In other words, introduction into a thickener/clarifier of an influent feed stream under high fluid velocity tends to cause turbulence in the tank and compromises the settling rate of solids. A feedwell may be structured in a variety of ways, therefore, to create or enhance dissipation of energy in the influent feed. For example, the feedwell and influent feed pipe may be structured to introduce influent feed to the feedwell at two opposing directions and into an annular space, such as is disclosed in U.S. Pat. No. 4,278,541 to Eis, et al.
In many feedwell assemblies, the influent feed pipe is incorporated into a dilution feed system including a mixing conduit with a downstream end connected to the feedwell and an upstream end with an eductor structure that delivers both a solids stream or slurry from a feed pipe and a diluting liquid from the clarified liquid at the top of the settling tank. The feed pipe is provided at its outlet end with a nozzle having a circular outlet opening located towards the upstream inlet end of the mixing conduit.
The mixing conduit may take the form of a classical submerged pipe or tube or alternatively an open channel form in which a mixing zone is open to the atmosphere. It has been observed that mixing of the incoming solids slurry with thickener overflow or dilution liquor is generally less complete or effective in the open channel design. It has been observed further that the dilution liquor stream tends to flow along the wall of the mixing channel, outside of the concentrated solids jet from the feed pipe nozzle, and only partially mixes with the concentrated solids jet. This type of performance is not ideal for a feed slurry dilution device that may also be mixing flocculant with a diluted slurry prior to entering a gravity thickener since a combined slurry stream entering the thickener or feedwell should be well flocculated and also diluted to a substantially uniform concentration of solids across the entire cross-sectional area of the mixing channel.